Electromagnetic variable force motors of the subject type include a pole piece having a central structure surrounded by a coil, an armature positioned for motion toward and away from the pole piece, and a spring for urging the armature away from the pole. The pole piece and armature are of ferromagnetic construction so that current in the coil establishes a magnetic field in the pole piece, attracting the armature toward the pole against the force of the spring. Sliding friction between the armature and surrounding structure, only partially reduced by armature guide and bushing structures, results in energy loss and position hysteresis between the armature and pole piece. Furthermore, armature and pole piece structures contoured to obtain a desired force characteristic relative to coil current are often complex and expensive to manufacture. One exemplary solenoid valve embodying a linear force motor of the described character is disclosed in U.S. Pat. No. 4,579,145.
U.S. Pat. Nos. 4,570,904 and 4,595,035, both assigned to the assignee hereof, disclose solenoid-operated modulating valves in which a ball of ferromagnetic construction serves as both the solenoid armature and the on/off valve element. A coil spring is captured within a central cavity in the pole piece and has an end tine which extends through a passage in the pole face to engage the ball and position the ball in normally-closing engagement with an opposing fluid passage valve seat spaced from the pole piece. The face of the pole tapers narrowingly in the direction of the ball for enhanced magnetic coupling therebetween. Fluid flow through the valve is controlled by pulse width modulation of the coil drive signal. Although the modulating valves so disclosed have enjoyed substantial acceptance and success, they often cannot substitute or satisfy requirements for variable force motor type solenoid valves.
It is an object of the present invention to provide an electromagnetic variable force motor which exhibits reduced friction between the armature and surrounding pole piece and housing structures, and thereby obtains both reduced energy loss and reduced hysteresis in armature movement.
Another object of the present invention is to provide a solenoid valve which embodies an electromagnetic variable force motor of the described character for enhanced precision control of fluids, such as hydraulic fluids in electronically controlled fluid transmissions.
A further object of the invention is to provide a solenoid valve of the described character which exhibits reduced size and complexity, and which is thus more economical to manufacture than are variable force motor solenoid valves of the prior art.